For transferring multi-valued information symbols, for example M-valued symbols (with M=2n), there exist in communications technology various (digital) modulation methods such as Frequency Shift Keying M-FKS, Phase Shift Keying M-PSK, or Quadrature Amplitude Modulation M-QAM. Such higher-level digital modulation methods enable the transfer of higher-valued information symbols, which can considerably increase the data transfer rate compared with the transfer of information symbols from an only low-valued (for example binary, i.e. M=2) symbol alphabet. In general the modulation of the data of higher-level-modulated signals demands a more complex receiver unit. Likewise, higher-level-modulated signals are more prone to disturbances such as noise or inter-symbol interferences (ISI), which results in a greater bit-error probability at the receiver.
Compared with simpler modulation methods such as the so-termed OOK modulation, (OOK=On-Off-Keying), the above-mentioned modulation methods (M-PSK, M-QAM, etc.) are more complex and in particular they require more energy for wireless data transfer. Whereas with FSK-, PSK- or QAM-modulations data to be transmitted are modulated on a permanently existing carrier, in contrast, with binary OOK modulation a carrier signal is only transmitted when, for example, a “1” is to be transferred: see FIG. 8a which shows a datum to be transmitted, FIG. 8b which shows an OOK signal resulting therefrom and FIG. 8c which shows a FSK signal corresponding to the datum. Thus, with OOK the information is modulated at the amplitude of the carrier signal (“0”=carrier off, “1”=carrier on). Accordingly, with OOK or on-off-keying energy is not required permanently for the transmission of information (for example, long “0” sequences need no energy). A disadvantage, however, is that in substantially weakened reception signals the strength can fall below a limit value so that a “1” is detected as a “0”. Thus, for stable data transfer with OOK it is used to use slower data transfer rates.
In order to be able with OOK or on-off-keying to transfer multi-valued information symbols, for the transfer so-termed spreading codes in the form of binary sequences can be transmitted, which can be demodulated for example by a correlation receiver. In this case each individual information symbol that has to be transmitted corresponds to a spreading code that comprises a plurality of bits (or chips). Use is made of the fact that there are particular binary sequences which, by virtue of good correlation results, can be recognized clearly by correlation modules despite disturbances. If there are several correlation stages in a receiver, then it is possible to distinguish more information symbols. For example, DE 10128353 A1 describes a method in which m spreading codes represent m symbols. In a corresponding receiver, for each of the spreading codes a corresponding correlation module is then needed in order to be able to detect an information symbol associated with the spreading code concerned. In a 2-valued or binary symbol alphabet (i.e. M=21), a first spreading or binary code can be associated with the first symbol (e.g. “0”) and a second spreading or binary code with the second symbol (e.g. “1”). Depending on the information symbol to be transmitted, the respective associated spreading codes can then be transferred from a transmitter to a receiver as an on-off-keying sequence, i.e. as an OOK-modulated binary sequence. On the part of the receiver it is then only necessary for a logic unit to count the interrupts (interruption commands) triggered by the spreading codes or OOK-modulated binary sequences.
However, with a 2-valued symbol alphabet only comparatively low data rates are possible. It is therefore desirable to transmit information symbols of a higher-valued symbol alphabet (M>2) by way of signal sequences associated with the information symbols that can be transmitted in an energy-efficient manner, and thereby at the same time to minimize the switching or signal processing complexity of a receiver.